The cooling of the superconducting elements requires the use of fluids at very low temperatures, generally below 5 K, these fluids forming sources of refrigeration placed in heat-exchange relationship with the components.
A fluid having a low boiling point, such as helium, allows such temperatures to be reached inside the storage tank where it is in a liquid/vapour equilibrium state.
Thus, it is possible, for example, to use the liquid phase contained in this tank as the source of refrigeration, by evaporating the liquid phase and optionally warming the gas thus produced.
This vaporization may take place inside the actual tank, the cold source then being isothermal and the corresponding refrigeration plant operating in a so-called refrigerating mode.
The vaporization may also take place outside the tank, after drawing off the liquid phase contained in the latter, the corresponding refrigeration plant then operating in a so-called liquefying mode. In this mode of operation, the gas thus produced is also used as the source of refrigeration, by warming it.
Finally, these two modes of operation may be combined in a mixed mode of operation, in which two sources of refrigeration are provided, namely a first source outside the storage tank and an isothermal second source actually inside this tank.
The production of refrigerating power at the required temperatures entails a high operating cost which corresponds to the compression requirements of the fluid used.